N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
4.5 Aromatic Hydrocarbons
103
Estimation of 13 C Chemical Shifts of Multiply Substituted Benzenes
and Naphthalenes (δ in ppm)
The 13 C chemical shifts of multiply substituted benzenes and naphthalenes (see
next pages) can be estimated using the substituent effects in the corresponding
monosubstituted hydrocarbons.
Example: Estimation of the chemical shifts for 3,5-dimethylnitrobenzene
Larger discrepancies between estimated and experimental values are to be expected
if the substituents are ortho to each other or if strongly electron-donating and electron-accepting groups occur simultaneously.
CH 3
CH 3
NO 2
1
2
3
4
C-1 base value
128.5
C-2 base value
128.5
Z 1 (NO 2 )
19.9
Z 2 (NO 2 )
-4.9
2 Z 3 (CH 3 )
-0.2
Z 2 (CH 3 )
0.7
estimated
148.2
Z 4 (CH 3 )
-3.0
exp
148.5
estimated
121.3
exp
121.7
C-3 base value
128.5
C-4 base value
128.5
Z 1 (CH 3 )
9.2
2 Z 2 (CH 3 )
1.4
Z 3 (CH 3 )
-0.1
Z 4 (NO 2 )
6.1
Z 3 (NO 2 )
0.9
estimated
136.0
estimated
138.5
exp
136.2
exp
139.6